A top defense attorney says the goal of any lawyer representing the Boston Marathon bombing suspect won't be to get him off, but simply to save his life.

"There doesn't seem to be a lot of questions as to factual guilt, at least from what you read in the newspaper and has been all over the TV," Thomas Anthony Durkin, a former United States Attorney whose work has included defending an associate of the 9/11 terrorists, told The Luke Burbank Show.

Federal prosecutors charged bombing suspect Dzhokhar Tsarnaev, 19, in his hospital room Monday with using a weapon of mass destruction to kill - a crime that carries a possible death sentence.

Durkin said as soon as an attorney is appointed, they'll likely start working on coming up with "mitigation factors" to convince a federal judge to deny the death penalty.

"He's young, that's a factor. He could very well be influenced by an older brother, which would be another factor. You know he's had an otherwise exemplary life," Durkin said.

It's a lengthy and costly process, especially since investigators will have to travel back to Russia to uncover as much as they can about Tsarnaev's past, just as prosecutors will do in their search for evidence and an explanation to what led the Tsarnaev brothers to allegedly set off the two bombs that killed three and injured nearly 200 others, some critically.

That cost means it's highly likely the task of defending Tsarnaev will fall on a public defender.

"It's going to require a lot of foreign travel, it's going to require an enormous amount of investigative effort," he said. "It would be an incredibly expensive proposition for a law firm to take on pro bono."

Some politicians had suggested Tsarnaev be tried as an enemy combatant in front of a military tribunal, where he would be denied some of the usual U.S. constitutional protections. But the Obama Administration said it had no choice but to prosecute Tsarnaev in the federal court system.

Tsarnaev, an ethnic Chechen from Russia who has lived in the United States for about a decade, is a naturalized U.S. citizen, and under U.S. law, American citizens cannot be tried by military tribunals, Durkin explained.

Durkin also called the intense debate about questioning Tsarnaev without informing him of his Miranda rights was a "red herring." The suspect was in custody for more than 60 hours before being told of his right to remain silent and to have an attorney provided to him.

The Justice Department invoked a public safety exception, which allows officials to bypass Miranda in the threat of imminent danger.

It's an issue that raises the hackles of many who fear the exception could be used to sidestep the constitution.

"It's a slippery slope to start fooling around with Miranda, but there are so many exceptions that I don't know how meaningful it is," Durkin said.

Durkin said it's likely Tsarnaev, as a recent U.S. citizen, was already well aware of his rights and the exception would have had little bearing on his willingness to cooperate - if he were medically capable. Tsarnaev remains in serious condition, reportedly suffering a throat injury preventing him from speaking.

As for the defense attorneys who end up representing Tsarnaev? Durkin said despite any outrage over the bombing, there's no moral quandary.

"That's our role, that's the role of the defense lawyer in the system and I frequently tell people if I had to like my clients I wouldn't be doing this," he said. "I'm in it because of the role that defense lawyers serve in our system."